linux-kernel - Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

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Date:   Thu, 5 Jan 2017 23:40:34 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve
 performance on some archs

On Thu, Jan 05, 2017 at 10:17:46AM -0500, Waiman Long wrote:
> On 01/05/2017 03:16 AM, Pan Xinhui wrote:
> >
> >
> > 在 2017/1/4 17:41, Peter Zijlstra 写道:
> >> On Tue, Jan 03, 2017 at 05:07:54PM -0500, Waiman Long wrote:
> >>> On 01/03/2017 11:18 AM, Peter Zijlstra wrote:
> >>>> On Sun, Dec 25, 2016 at 03:26:01PM -0500, Waiman Long wrote:
> >>>>> A number of cmpxchg calls in qspinlock_paravirt.h were replaced by
> >>>>> more
> >>>>> relaxed versions to improve performance on architectures that use
> >>>>> LL/SC.
> >>>> Claim without numbers ;-)
> >>>
> >>> Well it is hard to produce actual numbers here as I don't have the
> >>> setup
> >>> to gather data.
> >>
> >> Surely RHT has big PPC machines around? I know that getting to them is a
> >> wee bit of a bother, but they should be available somewhere.
> >>
> > hi,
> >
> > I do some tests about cmpxchg and cmpxchg_acquire before on ppc.
> >
> > loops in 15s of each cmpxchg is below.
> >
> > cmpxchg_relaxed: 336663
> > cmpxchg_release: 369054
> > cmpxchg_acquire: 363364
> > cmpxchg:     179435
> >
> > so cmpxchg is really expensive than others.
> > but I also have doubt about the cmpxchg_relaxed, it should be the
> > cheapest, but from the tests, release/acquire are faster than it.
> >
> > thanks
> > xinhui
> >
> Thanks for doing the test. It looks like we should just focus on using
> either cmpxchg_release or cmpxchg_acquire and forget about cmpxchg_relaxed.
> 

_relaxed is more lightweight in two aspects:

1.	_relaxed is not a compiler barrier, so it allows compilers do
	more optimizations than _acquire and _release.

2.	_relaxed doesn't introduce HW barriers and doesn't performs
	worse than _acquire and _release in _most_ _usual_ cases.   

And on some ll/sc archs without a lightweigt barrier like "lwsync" for
_acquire and _release, #2 could be more significant.

So don't hesitate to use _relaxed whenever it's possible and reasonable
;-)

Regards,
Boqun

> Cheers,
> Longman
> 

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